I'm working (slowly) on a project to build a curve tracer based largely on the Heathkit original (Heathkit Catelogue - IT-1121) and I've come up against an issue in the step generating circuit.
The issue is with the step reset logic which produces a spike at the end of the selected sequence of steps. It took me a while to twig what the issue was, but the basis of the design is that the reset circuit "sees" the next step before the reset is triggered. The consequence of this is that the smallest spike of the n+1 step is produced at the output.
Clearly I can't have these spikes messing up my DUT base/gate signal and I'm assuming this issue existed in the original. To my annoyance, I missed this at the breadboard stage (I did notice the spikes, but I put it down to stray inductance). Right now I'm thinking I should re-build the section completely, most likely using a microprocessor. But, before I (reluctantly) abandon this board, can anyone suggest an alteration that would remove the spikes? One thought I had was to reset using the NOR gate pulses - so instead of looking at the output, I count the input pulses and reset on the rising edge (given that the counter IC increments on the falling edge).
Note: careful observers will notice that I've altered the reset circuit slightly to avoid having to use the (impossible to obtain 7M5 POT), but I'm pretty sure the update retains the original idea.